Nicomedia (^{[pronunciation?]}; Greek: Νικομήδεια, Nikomedeia; modern İzmit) was an ancient city in what is now Turkey, founded in 712/11 BC as a Megarian colony and was originally known as Astacus (Ancient Greek: Αστακός, "lobster"). After being destroyed by Lysimachus, it was rebuilt by Nicomedes I of Bithynia in 264 BC under the name of Nicomedia, and has ever since been one of the most important cities in northwestern Asia Minor. Hannibal came to Nicomedia in his final years and committed suicide in nearby Libyssa (Diliskelesi, Gebze). The historian Arrian was born there.

Nicomedia was the metropolis of Bithynia under the Roman Empire, and Diocletian made it the eastern capital city of the Roman Empire in 286 when he introduced the Tetrarchy system. Nicomedia remained as the eastern (and most senior) capital of the Roman Empire until co-emperor Licinius was defeated by Constantine the Great at the Battle of Chrysopolis (Üsküdar) in 324. Constantine mainly resided in Nicomedia as his interim capital city for the next six years, until in 330 he declared the nearby Byzantium (which was renamed Constantinople) the new capital. Constantine died in a royal villa in the vicinity of Nicomedia in 337. Owing to its position at the convergence of the Asiatic roads leading to the new capital, Nicomedia retained its importance even after the foundation of Constantinople.